It is common practice when measuring the surface contour of an object to take measurements of the object using an optical or laser measuring apparatus. One such method includes marking predetermined locations on the object and attaching targets to the object at each location. Conventional target bases generally utilize a magnet or multiple suction cups to attach the target to the object. Conventional target bases may also provide proper spacing of the target from the surface of the object by feet that make point contact with the object. In many applications, it is necessary to keep the targets attached to the object being measured for long periods of time, such as when two or more sub-assemblies are being assembled and their relative positions are being established prior to joining them.
Conventional target bases that utilize small suction cups generally operate by being pressed onto the surface of the object. The air is squeezed out of the suction cups and the suction cups are released whereupon they partially resile and form a vacuum within the suction cup. Suction cups have several disadvantages. For example, the force that the suction cups exert is relatively small, inasmuch as it is a function of the resiliency of the suction cup. Accordingly, even when several suction cups are used on a target, the force holding the target to the object is relatively low, and there is a chance that the target will change position or even become dislodged. In addition, suction cups must be applied with attention to making sure that all of the suction cups are evenly pressed onto the surface of the object, lest there be an uneven suction force among them, thus allowing the target to tilt in the direction of a greater suction force. Furthermore, suction cups are inherently resilient, thereby displacing the target away from the surface. Because of the displacement, or rebound, of the target from the surface of the object, the legs that set the distance of the target from the surface of the object are often not effectively engaged.
The amount of rebound the target experiences is subject to variation that is dependent upon many factors, such as environmental conditions (e.g., temperature and humidity), the orientation of the target (e.g., gravity forces opposing or aiding the suction cups), and the physical condition of the suction cups (e.g. age, wear, and the like). Other problems associated with suction cups may include creep of the sealing lips of the suction cups that allows them to loosen, and leakage that allows the suction cups to lose their vacuum over time.
In short, pressed-on suction cups provide scant assurance that the targets on the object are uniformly positioned at predetermined locations and that the offset distance and the target angle accurately reflects the surface of the object. In addition, it cannot be assured that pressed-on suction cups will not move over time or even be dislodged.